Low reflectance chemical agent resistant coating compositions

ABSTRACT

A powder coating composition comprising:
         (a) a first acrylic powder resin having a hydroxyl value of at least about 180;   (b) a second acrylic powder resin having a hydroxyl value less than about 45;   (c) a crosslinker reactive with the hydroxyl functional acrylic resins; and   (d) wollastonite.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional of co-pending U.S. application Ser. No.12/544,510 filed Aug. 20, 2009, which claims priority to U.S.Provisional Ser. No. 61/090,446 filed on Aug. 20, 2008, the entirety ofboth of which are hereby incorporated by reference.

This invention relates to powder coating compositions having excellentchemical resistance and extremely low gloss. These properties areobtained by the selection of a combination of hydroxy functional acrylicpowder resins having different hydroxyl values and by the use ofwollastonite as a flattening pigment.

Very low gloss powder coatings are desirable for a variety of usesincluding automotive interior parts, wheel rims, bumpers, firearms, andmilitary applications and other articles where low reflectance isdesired. A particular requirement of some military applications involvesresistance to penetration by chemical warfare agents according tomilitary specifications such as MIL-C-46168. For some military andcommercial applications it is useful to provide a coating compositionwhich when applied to a substrate and cured will have a 60° gloss lessthan 1 and an 85° gloss less than 7. Although a number of prior artapproaches have taught systems which can provide very low gloss powdercoatings, there is a need to provide low gloss powder coatings whichalso provide resistance to chemical warfare agents.

This invention therefore relates to a low gloss chemical agent resistantpowder coating composition comprising:

-   -   (a) a first acrylic powder resin having a hydroxyl value of at        least about 180;    -   (b) a second acrylic powder resin having a hydroxyl value less        than about 45;    -   (c) a crosslinker reactive with the hydroxyl functional acrylic        resins; and    -   (d) wollastonite.        For certain embodiments of this invention it is useful to have        the weight ratio of the first acrylic powder resin to the second        acrylic powder resin be greater than 1.0,

1. Hydroxy Functional Acrylic Powder Resin

One aspect of this invention involves the selection of two differenthydroxyl functional acrylic powder resins having varying hydroxylvalues. The production of acrylic polymers of a given hydroxyl number iswell known within the art. The hydroxyl functional acrylic polymers ofthis invention are obtained in a customary manner, for example, bypolymerizing at least one hydroxyl functional monomer, typically ahydroxyl functional (meth)acrylate such as hydroxyethyl or hydroxypropyl(meth)acrylate, along with other copolymerizable unsaturated monomers.Representative copolymerizable monomers include vinyl compounds such asstyrene, methylstyrene, vinyl acetate, and the unsaturated alkyl estersof (meth)acrylic acids. Suitable alkyl (meth)acrylate monomers include,for example, methyl (meth)acrylate, ethyl (meth)acrylate, butyl(meth)acrylate, propyl (meth)acrylate, 2-ethyl hexyl (meth)acrylate,cyclohexyl (meth)acrylate, decyl (meth)acrylate, isodecyl(meth)acrylate, benzyl (meth)acrylate, isobornyl (meth)acrylate,neopentyl (meth)acrylate, and 1-adamatyl methacrylate. By selecting thelevel of hydroxyl functional monomer which is incorporated into theacrylic powder resins, any given hydroxyl value can be selected. Thepolymers can be formulated to any useful glass transition temperature.For some applications it is useful to have a Tg of at least 35° C., andsometimes at least 40° C. for each of the hydroxy functional acrylicresins. For some applications it is useful to have the resin with thehigher hydroxyl number have a Tg of at least 45° C. or higher to provideever greater resistance to chemical agents.

For this invention, a first hydroxyl functional acrylic powder resinwill be produced having a hydroxyl value of at least about 180, and asecond hydroxyl functional acrylic polymer resin will be produced havinga hydroxyl value less than about 45. These resins are then combined witha suitable crosslinking agent reactive with hydroxyl groups, such as ablocked isocyanate or uretdione, to provide the powder coatingcomposition. For some embodiments of this invention, the first acrylicpowder resin will have a hydroxyl value between about 180 and about 250,and for some embodiments of this invention it is useful for the secondacrylic powder resin to have a hydroxyl value between about 5 and 45 andfor some embodiments between about 20 and 45.

For those applications requiring resistance to chemical agents it isoften useful to provide a weight ratio of the first acrylic powder resinto the second acrylic powder resin greater than 1.0. Typically, for thecombination of the first acrylic powder resin and the second acrylicpowder resin, between about 50.1 weight percent and 85.0 weight percentof the total amount of hydroxy functional acrylic resin would be thefirst acrylic powder resin, For some applications, between about 50.1and 60 percent by weight of the combined weight of the first and secondacrylic resins would be the first acrylic powder resin.

The powder coatings of this invention can be produced using conventionalpowder coating production methods, but it has been found that for someapplications it is preferred to process the portion comprising the firstacrylic powder resin and the portion comprising the second acrylicpowder resin separately at first and then combining the separateportions into a final coating.

This process comprises:

-   -   (a) extruding and grinding a first powder composition comprising        a crosslinker and a first acrylic powder resin having a hydroxyl        value of at least about 180; and    -   (b) extruding and grinding a second powder composition        comprising a crosslinker and a second acrylic powder resin        having a hydroxyl value less than about 45;    -   (c) combining the ground, extruded first and second powder        compositions to provide a weight ratio of the first acrylic        resin to the second acrylic resin greater than 1.0.

Additionally, it has been found that the use of wollastonite as aflattening pigment is useful in meeting the very low gloss and chemicalresistance requirements of these coatings. Typically, the wollastonitewould be present at a weight solids percent of at least 10 percent andfor some embodiments, at least 15 percent, and for some embodiments atleast 25 percent of the total weight solids of the coating. For someapplications it is useful to use a level of wollastonite between about10 and 35 percent by weight of the total weight solids of the finalpowder coating composition comprising the resins, the crosslinker andpigments.

Additional pigments such as titanium dioxide, metallic pigments, ironoxides, carbon black, organic pigments etc. can also be included. Thecoatings may also contain extender pigments, including polymericextender pigments provided they are stable at the processing and curetemperatures of the powder coatings.

Additives such as flow agents, degassing agents, antistatic. agents,plasticizers, light stabilizers, light absorbers, catalysts etc. canalso be added.

The powder coatings of this invention can be prepared by typical meanswell known in the art such as by first melt blending the ingredients ofthe coating compositions in an extruder at a suitable temperature. Theextrudate is then cooled and pulverized. In one useful process for thisinvention, the first acrylic powder resin and a suitable crosslinker areextruded and ground or pulverized as one composition and the secondacrylic powder resin and a crosslinker are separately extruded andground or pulverized. These dried materials can then blended into afinal coating composition.

The application of the powder coatings can be made by any means wellknown in the art for powder coatings such as by electrostatic sprayingor by the use of a fluidized bed to any suitable substrate. If desired,the substrate can optionally be preheated prior to application of thepowder coating composition. Once the coating composition has beenapplied to the substrate, the coating is cured by heating at atemperature and for a length of time sufficient to cause the reactantsto form an insoluable polymer network. Typical cure temperatures wouldnormally range from about 125° C. to about 250° C. for a period of about10 to about 30 minutes. Suitable substitutes include any substrate whichis not adversely affected by the cure time and temperature and wouldtypically include metal, glass and plastic substrates. If desired, thesubstrate can be pretreated and/or coated with one or more suitableprimer coats before application of the coatings of this invention.

The following examples have been selected to illustrate some specificembodiments and practices of advantage to a more complete understandingof the invention. Unless otherwise stated, “parts” means parts-by-weightand “percent” is percent-by-weight, and equivalent weight is on a weightsolids basis.

Examples 1-10 in Table 1 were prepared by blending and extruding thelisted raw materials. The extrudate was cooled and fractured to producea powder coating which was then applied to a suitable substrate andbaked for 10 minutes at 400°F. and then tested for gloss and otherproperties. Chemical agent resistance testing results were generallymore favorable for coatings having a weight ratio of resin 1/resin 2greater than 1.0.

TABLE 1 RAW MATERIAL EX. #1 EX. #2 EX. #3 EX. #4 EX. #5 EX. #6 EX. #7EX. #8 EX. #9 EX. #10 Hydroxy 237.0 220.0 214.0 208.0 202.0 196.0 176.0214.0 208.0 202.0 functional acrylic1 Hydroxy 158.0 188.0 198.0 208.0218.0 230.0 264.0 198.0 208.0 218.0 functional acrylic2 Blocked 256.0203.0 239.0 234.0 231.0 226.0 211.0 239.0 234.0 231.0 iscyanate3 Flowagent4 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 Benzoin 5.0 5.05.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 Wollastonite5 184.0 184.0 184.0 185.0184.0 183.0 184.0 174.0 175.0 174.0 Polymeric 10.0 10.0 10.0 extender6Cobalt 62.7 62.7 62.7 62.7 62.7 62.7 62.7 62.7 62.7 62.7 chromic greenpigment Chrome oxide 81.0 81.0 81.0 81.0 81.0 81.0 81.0 81.0 81.0 81.0green pigment Iron oxide red 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3pigment Gloss 60/85 2.2/6.7 0.7/4 6 0.7/5.5. 0.6/5.0 0.5/5.8 5/5.10.5/5.5 1Isocryl ™ H-280 commercially available acrylic powder resinfrom Estron Chemicals having a hydroxyl number of 200, an eq. wt. of280, a specific gravity of 1.14, a Tg of about 42° C. and a softeningpoint of 105° C. 2Acrylic powder resin having a hydroxyl number of 30,an eq. wt. of 1870, a specific gravity of 1.11, is Tg of about 50° C.and a softening point of 112° C. 3Vestagon ™ B 1530 e-caprolactamblocked IPD1 commercially available from Degussa 4Resiflow ™ PL-200acrylic flow agent commercially available from Estron Chemicals5Nyglos ™ 4W commercially available from Nyco Minerals 6Pergopak ™ M4urea formaldehyde beads commercially available from Albemarle Corp.Examples 11-13 in Table 1 were prepared by blending and extruding thelisted raw materials. The extrudate was cooled and fractured to producea powder coating which was then applied to a suitable substrate andbaked for 10 minutes at 400° F. and then tested for gloss and otherproperties. As a comparison, the products of Examples 12 and 13 werecombined in equal weight amounts, and that powder coating was appliedand tested in the same manner. That example produced a 60/85 gloss ofonly 0.5/5.0.

TABLE 2 RAW MATERIAL EX #11 EX #12 EX #13 Hvdroxy functional acrylic7208.0 320.0 Hydroxy functional acrylic8 208 552.5 Blocked isocyanate9234 315 82.5 Flow agent10 10.0 10.0 10.0 Benzoin 5.0 5.0 5.0Wollastonite11 185 210 210.0 Cobalt chromic green pigment 62.7 62.7 62.7Chrome oxide green pigment 81.0 81.0 81.0 Iron oxide red pigment¹² 6.36.3 6.3 Gloss 60/85 11.8/21.6 10.6/45.4 51.3/84.4 7Acrylic powder resinhaving a hydroxyl number of 200, an eq. wt of 280 and a Tg of about 63°C. 8Acrylic powder resin having a hydroxyl number of 30, an eq. wt. of1870, a specific gravity of 1.11, a Tg of about 50° C. and a softeningpoint of 112° C. 9Vestagon ™ B 1530 e-caprolactam blocked IPD1commercially available from Degussa 10Resiflow ™ PL-200 acrylic flowagent commercially available from Estron Chemicals 11Nyglos ™ 4Wcommercially available from Nyco Minerals ¹²In Examples 12 and 13 theiron oxide red pigment was added as 63 gross of a 10% solids slurry.

While this invention has been described by a specific number ofembodiments, other variations and modifications may be made withoutdeparting from the spirit and scope of the invention as set forth in theappended claims.

We claim:
 1. A powder coating composition comprising: (a) a firsthydroxyl functional acrylic powder resin having a hydroxyl value of atleast about 180; (b) a second hydroxyl functional acrylic powder resinhaving a hydroxyl value less than about 45; (c) a crosslinker reactivewith the hydroxyl functional acrylic resins; and (d) wollastonite; andwherein the weight ratio of the first acrylic powder resin to the secondacrylic powder resin is greater than 1.0.
 2. The powder coating of claim1 wherein the coating composition when applied to a substrate and curedwill have a 60° gloss less than 1 and an 85° gloss less than
 7. 3. Thepowder coating of claim 1 wherein the first acrylic powder resin has ahydroxyl value between 180 and
 250. 4. The powder coating of claim 1wherein the second acrylic powder resin has a hydroxyl value between 20and
 45. 5. The powder coating of claim 1 wherein the wollastonite ispresent at a level of at least 10% by weight solids of the coatingcomposition.
 6. The powder coating of claim 1 wherein the wollastoniteis present at a level of at least 15% by weight solids of the coatingcomposition.
 7. The powder coating of claim 1 wherein the wollastoniteis present at a level of at least 25% by weight solids of the coatingcomposition.
 8. The powder coating of claim 1 wherein the first acrylicpowder resin and the second acrylic powder resin each have a Tg of atleast 35° C.
 9. The powder coating of claim 1 wherein the first acrylicpowder resin has a Tg of at least 45° C.
 10. The powder coating of claim1 wherein the first acrylic polymer is present at a level to providebetween about 50.1 weight percent and 85.0 weight percent of the totalcombined weight of the first acrylic polymer and the second acrylicpolymer.
 11. The powder coating of claim 1 wherein the first acrylicpolymer is present at a level to provide between about 50.1 weightpercent and 60.0 weight percent of the total combined weight of thefirst acrylic polymer and the second acrylic polymer.
 12. A powdercoating composition comprising: (a) a first hydroxyl functional acrylicpowder resin having a hydroxyl value of at least about 180 and having aTg of at least 35° C.; (b) a second hydroxyl functional acrylic powderresin having a hydroxyl value less than about 45 and having a Tg of atleast 35° C.; (c) a crosslinker reactive with the first hydroxylfunctional acrylic powder resin and the second hydroxyl functionalacrylic resin; and (d) wollastonite; and wherein the weight ratio of thefirst acrylic powder resin to the second acrylic powder resin is greaterthan 1.0 and wherein the wollastonite is present at a level of at least10% by weight of the coating composition.
 13. A process for producing alow gloss powder coating which process comprises: (a) extruding andgrinding a first powder composition comprising a crosslinker and a firstacrylic powder resin having a hydroxyl value of at least about 180; and(b) extruding and grinding second powder composition comprising acrosslinker and a second acrylic powder resin having a hydroxyl valueless than about 45; and (c) combining the ground, extruded powdercomposition to provide a weight ratio of the first acrylic resin to thesecond acrylic resin greater than 1.0.
 14. The process of claim 13wherein at least one of the first powder composition or the secondpowder composition comprises wollastonite.
 15. A process of coating asubstrate which process comprises: (i) applying a powder coatingcomposition to a substrate; and (ii) heating the coated substrate at atemperature and for a length of time sufficient to cause the compositionto form an insoluble polymer network; and wherein the powder coatingcomposition comprises: (a) a first hydroxyl functional acrylic powderresin having a hydroxyl value of at least about 180; (b) a secondhydroxyl functional acrylic powder resin having a hydroxyl value lessthan about 45; (c) a crosslinker reactive with the hydroxyl functionalacrylic resins; and (d) wollastonite; and wherein the weight ratio ofthe first acrylic powder resin to the second acrylic powder resin isgreater than 1.0.
 16. A substrate coated by the process of claim 15.